Field of the Invention
The present invention relates to an illumination optical system and an image projection apparatus, such as a liquid crystal projector.
Description of the Related Art
Japanese Patent Laid-Open No. (“JP”) 2008-299298 proposes changing of a compression ratio of a light flux between a longitudinal section and a lateral section in an illumination optical system for a liquid crystal projector where the longitudinal section is a plane that contains a normal of a polarization splitting surface and the lateral section is a section that contains the optical axis and is orthogonal to the longitudinal section. The compression of the light flux on the longitudinal section is effective in narrowing an incident angle distribution of a light flux upon the PBS and to maintain the contrast. The compression of the light flux on the lateral section is effective in a miniaturization of the illumination optical system.
JP 7-181392 provides a first integrator having a positive power and a second integrator having a negative power on the lateral section. JP 2002-40416 compresses a light flux using a cylindrical lens rather than an integrator on the longitudinal section, and does not compress the light flux on the lateral section.
However, JPs 7-181392 and 2002-40416 are disadvantageous because they compress the light flux only in a single direction, that is, either on the longitudinal direction or on the lateral direction, and the illumination optical system (or the liquid crystal projector) becomes longer.
In addition, the illumination optical system disclosed in JP 2008-299298 secures the power of the first integrator on the lateral section by changing (or decentering) a height of a vertex of each lens cell of the first integrator. Therefore, a distance between each lens cell and the polarization converter does not become constant, and a sufficiently bright image cannot be provided.